This invention relates generally to photosensitive devices and in particular to the use of microlens arrays to increase the fill factor of such photosensitive devices.
Currently, surface mount packaging is enjoying considerable commercial acceptance in large part due to the fact that high volume connections may be made to circuit boards in a low cost fashion. In one such surface mount integrated circuit packaging technique, known as Quad Flat Package (QFP), the package may be connected to a circuit board, such as a printed circuit board (PCB), simply by positioning the package on the board and applying heat. The heat melts a plurality of contacts on one surface of the package, connecting the package to the board. This technique enables integrated circuits to be connected to circuit boards in an automated fashion at high speed and low cost.
Integrated circuit image sensors are also gaining increasing acceptance. Complementary metal oxide semiconductor (CMOS) imaging devices may be formed using conventional logic semiconductor fabrication processes. These devices can be made at high speed and at relatively low cost, producing devices with advanced electronic functions, in addition to the imaging functions.
A variety of photosensitive devices increase their fill factors with microlens arrays. The fill factor is a measure of the amount of incident light which actually makes its way onto the image sensing array. The microlens operates as a s miniature lens, focusing light on each pixel making up the image sensing array.
Microlenses may be made of positive photoresist or sol gel. These microlens arrays may be formed directly on top of the image sensor, for example, by positioning the microlens array on top of a color filter array (CFA) which is situated over the image sensing array. Alternatively, the microlenses may be spaced over the image sensing array. In any case, the microlenses are generally heat sensitive and therefore have not been used, so far as the present inventors are aware, with surface mount packaging. It is believed that conventional surface mount packaging, which involves temperatures on the order of 225xc2x0 C. for a matter of minutes adversely affects conventional microlens arrays.
Therefore, there is a continuing need for techniques for enabling low cost packaging techniques to be applied to photosensitive devices with acceptable fill factors achieved using microlens arrays.
In accordance with one aspect, a method for forming a microlens includes forming a photosensitive device. A microlens is arranged over the device and the device is packaged. The packaged device is then exposed to a surface mount mass reflow.